Doing It for the Art: Tennis was meant to be a Sonic-only game, but Sumo believed it would be more interesting to use various SEGA franchises. And as their philosophy to recreating some games which haven't been seen in a long time is 'what you remember most about them', this results in a TON of fanservice and attention to detail. While they admit they do keep turning a profit in mind, they also look at what franchises can bring to the table to create a fun and memorable game.
And both fans and SEGA have been impressed by their passion; it was reported that when the team showed their recreation track of Skies of Arcadia in Transformed to the game's original producer, she came close to tears due to how faithful and nostalgic the result was.
Screwed by the Network: Since Samba De Amigo relied heavily on licensed music, tracks were limited for the stages in the racing games. In the first All-Stars Racing, the songs consisted of ones done by Bellini and Wave Master (the latter being Sega's in-house composing team). Most of Bellini's music was never in a Samba de Amigo game, and one of Wave Master's songs only appeared in the Japanese only Mission Pack Sequel. Transformed removed the Bellini songs altogether.
What Could Have Been: The game originally started off as a Wii version of Virtua Tennis. Sumo Digital made the game more cute to appeal to the casual audience. However, it didn't look right. When SEGA saw this, they recommended they made the game around Sonic. While Sumo liked this idea and agreed, they also suggested looking at including characters from all of SEGA's IPs. And the rest is history.
Likewise, Svend "Dreadknux" Jocelynde of the Sonic Stadium wrote a story behind the game, which was also scrapped. His treatment stated that Eggman entered the tournament in order to benefit his plans for world domination, others wanted fame or objects of value to them, while the main Sonic cast (minus Eggman) just want to stop Eggman.
Magical Sound Shower Explanation Music track in the classic 1986 OutRun
Red Out Explanation Music track in After Burner III
Rolling Start Explanation Alternate name for "The King of Speed" in Daytona USA
True Blue Explanation A Sonic the Hedgehog compilation album most notable for featuring the first complete Super Smash Bros. Brawl music track (The Angel Island remix) beforeBrawl's release; considering how titanic the hype for that game was, well...
Working Man Explanation Shenmue forklift theme. The achievement itself requires the forklift.
Achievements named after miscellaneous Sega concepts:
Toe Jam And Earl could have been in the game if Sega got proper licensing with the series' developers in time. They couldn't. They aren't in the sequel either, and even though they got lots of votes for the sequel's DLC poll, it's highly unlikely that they will be added in.
Depending on sales, Downloadable Content could have made these possibilities a reality, but the developer has since revealed that Sega only contracted them to create Metal Sonic, so nothing further will come out. However, a sequel was made.
A Space Channel 5 track was in the pipeline, but got cut because the dev team weren't impressed with it's quality.
Weapons from Puyo Pop were originally in the game, but removed.
Ulala: A portion of "The Reporter from Space Channel 5".
Pudding: A portion of "Guitar Showdown" from Space Channel 5: Part 2.
General Winter: The Ominous Russian Chanting of "Don't Weep, That Time Has Passed" from Company of Heroes 2 (and we mean only the chanting, which was extracted seperately from the original source music for this game).
Mii: This very game's matchmaking music, for some reason.
Danica Patrick and Football Manager: Original music, because Danica is a real-life person and Football Manager has literally no music to speak of.
Sonic, Tails, Knuckles, Amy Rose & Shadow all have new voice actors following the majority of the Sonic the Hedgehog characters being recast after the first All-Stars Racing was released in 2010.
Vyse manages to avert this trope whilst playing it straight at the same time - Christopher Sullivan reprises his role from the original game over a decade earlier; replacing Roger Craig Smith, who had voiced Vyse during his cameo in Valkyria Chronicles.
A Daytona USA track was considered (specifically Three Seven Speedway), but was scrapped as SUMO realised it wouldn't take advantage of the transformation mechanic, and was subsequently replaced with an Out Run track.
Orta made it to the shortlist, but concerns arose about vehicle concepts with wings clipping through other characters and she was dropped.
Billy Hatcher was planned, but was dropped because SUMO couldn't decide on a design for his car. Time ran out and they were forced to not include him. It is said that his incomplete files exist on the game's disc. In hindsight, this would explain why he doesn't even have a cameo in neither Race of Ages alongside the other removed racers of the previous game nor in the track of his game alongside his friends.
The executive developer stated that had they asked Nintendo for an exclusive character for the Wii U version of the game, his personal choice would have been Kirby.
Classic Sonic was considered, complete with concept art and vehicle designs. However, SEGA said no to Sumo because they didn't want Classic Sonic to appear alongside Sonic in a game without a specific time travel explanation.
Aika was on the original shortlist, but because she lacked a model for the crew to go from unlike other secondary characters such as Pudding and Reala (since they were in Sega Superstars Tennis) she was cut.